The present invention pertains to a process and a device for the detection of the position or register position of a paper web, especially in wet offset printing.
Newspapers are produced predominantly according to the offset process. A plurality of paper webs are wound off from rolls, printed in the printing units and finally folded in the folding apparatus and cut. While one paper web is running through the press, it is continuously in a stretched state. Paper comes in contact with water and ink during wet offset printing, whereby the stretching properties of the papers are changed. The paths over which the individual paper webs run have different lengths. The addition of water and ink is not the same for all printing mechanisms. Various guide elements are used in order to turn the webs and to optimize the run of the web. The stretching properties also vary between different grades of paper and different paper weights, and there are tolerances in the elastic properties of the paper from one roll to the next and even within one roll even within the same grade of paper.
The printed webs are first folded into bundles in the folding apparatus, and the bundles are subsequently cut such that the cut is located outside the printing area of the pages. Errors in the cutting position make it impossible to sell the newspapers. The cutting position can be adjusted by the printer by adjusting the web length between the printing mechanism and the cutting position by means of so-called compensator rollers. It is also possible to set the cutting position by an equal adjustment of the printing positions of all printing mechanisms and thus to do away with a register roller (which is also called virtual main crop mark). A separate compensator roller is necessary for one half of the divided web in the case of double-width webs that are divided and placed one over another before the entry into the folding apparatus. The so-called secondary register is thus set.
Difficulties arise during the correct cutting of the bundled webs in the folding apparatus from the different paths of each web and the different stretchings, which the paper webs undergo during the run through the printing press. When similar productions are repeated, the cutting position is approximately known and can be roughly preset from the beginning. In fact, the printer must, however, accurately set the cutting position at the beginning of each production. This results in a certain number of spoiled copies and a loss of time during the production.
Processes that make possible the rapid, automatic regulation of the cutting position have been known. Measuring fields of various sizes are printed along, which are detected by suitable sensors shortly before the folding apparatus and thus provide a measuring reference, which can be used to regulate the cutting position. A large measuring mark, which is placed at the edge of the page that is not used for printing, can be detected, e.g., by a photodiode, but it compromises the overall visual impression. Smaller marks can be recognized by video cameras in conjunction with a digital pattern recognition. The drawback of both processes is that additional marks must be printed along. The technical effort for the recognition of these marks increases, in general, with decreasing size of these marks.
Another possibility of recognizing errors in the cutting position has been known from DE 199 10 835 C1, in which the printed image in two areas are scanned. The first scanning at the outlet of the printing mechanism forms the reference, with which the scanning before the entry into the folding apparatus is compared. The position errors can be determined from the correlation of the signals and used for the regulation. The drawback of this process is that a plurality of sensors are necessary and these must be placed suitably.
One object of the present invention is to provide a process and a device especially for wet offset printing, which are able to automatically recognize the position or register position.
According to the present invention, the position or register position of a paper web, especially in wet offset printing, is detected by detecting strips of the paper web that are free from moistening agent. In addition or as an alternative, it is also possible to detect a strip in which there is no printing ink.
The device according to the present invention for detecting the position of a paper web, especially in wet offset printing, has a sensor, with which a moistening agent, e.g., water, or the water content in a paper web can be detected. According to a second embodiment, the same sensor or another sensor is used to detect printing ink on the paper web. An evaluating unit, with which the sensor signals can be evaluated and the moistening agent content of a paper web can be detected, is preferably provided.
According to another aspect, the present invention pertains to the regulation of the position of a paper web using the detected position, so that the cutting length of the paper web can be set, e.g., based on the detected printing ink-free or moistening agent-free strip, e.g., by the automatic actuation of a compensator roller.
The principle of function of the present invention will be described below.
Wet offset printing is based on the different surface tensions of the moistening agent and the printing ink. The process has been described several times and shall not be explained in greater detail here. The only thing that should be referred to here is the circumstance that water also comes into contact with the web besides the printing ink during the printing on the paper web, doing so on the area that approximately corresponds to the printing area. However, no water is transferred to the paper web in the area of the clamping device of the printing plates. The amount of moisture entering the paper depends on the number of printing mechanisms that come into contact with the web and, in addition, on the amount of moistening agent that is used in the particular printing mechanism. A page of a newspaper is usually printed on the front and the back. Consequently, depending on the number of colors used, 2 to 8 printing mechanisms are in contact with one paper web. Since each printing mechanism is set in good register, the zone free from printing ink or moistening agent forms a strip across the web, which is always located in the same area, for all printing mechanisms.
The present invention is based on the recognition of this printing ink-free or moistening agent-free strip. In fact, this area, which will hereinafter be called a channel strip, forms in a certain way an invisible marking or mask. Even though the marking formed by the moistening agent-free strip disappears after a few minutes, is it nevertheless present shortly after the printing operation and can be used to recognize or regulate the position.
The moistening agent consists mainly of water. Water is transparent, i.e., visible light is hardly absorbed by water. By contrast, water shows a marked absorption for some wavelengths in the infrared range and in the range close to the infrared. At a wavelength of 2.95 xcexcm, water has an absorption, at which the depth of penetration for electromagnetic radiation is only about 1 xcexcm, i.e., if such infrared light passes through a layer of water with a thickness of 1 xcexcm, more than 60% of the radiation is absorbed. Less marked absorption maxima are located at half and one third of this wavelength. Furthermore, additional absorption peaks are found for water at the harmonic multiples of the vibration frequency of the electromagnetic radiation of the wavelength of 2.95 xcexcm. For example, it is possible to measure absorption maxima at about 1.48 xcexcm, 980 nm, etc. Even though the absorption is less intense there, faster and more sensitive detectors are available at these shorter wavelengths. In addition, there are powerful light sources, e.g., semiconductor lasers, which can be advantageously used for the measuring device. The semiconductor materials that can be used for detectors or light sources include, e.g., InGaAsP, Ge, Si, AgOCs, and HgCdZnTe (also known under the name MCT).
It is also possible to measure this absorption behavior in a reflection set-up. The process will be hereinafter described for the reflection measurement, but the present invention may be used in the case of transmission measurement as well.
The optical detection of water can be performed if, e.g., a radiation source for a spectral range of 2.8-3 xcexcm and a corresponding sensor are used. If the web is illuminated by such a radiation source and the remitted radiation is measured, a lower remission is obtained for paper containing water than for dry paper. If a spot measurement is performed and, e.g., the printed paper web is moving under a stationarily arranged measuring head, a maximum signal will always be obtained at the moments at which the unprinted channel strip passes through at the measurement point. The measured area should be narrower than the width of the channel strip in order to obtain a sharp measured signal. For example, 10-20 mm may be assumed to be a typical width of a channel strip. The shape of the measured area is of secondary importance; it maybe, e.g., round or elliptical, square or rectangular or of any other shape, but it should preferably have a width of less than 5 mm and should be advantageously arranged such that its narrow side is located in the direction in which the web runs, whereas the longer extension is in the direction of the channel strip to be detected, i.e., at right angles to the run of the web. For example, the light of the radiation source may be focused for such a spot measurement, or the radiation emitted by a measured area may be focused on a detector, or the field of view of the radiation source and/or detector may be limited by means of appropriate diaphragms.
The absorption or reflection of electromagnetic radiation in the printing ink can be measured to detect the ink-free strip. If a paper web is scanned, e.g., continuously, a reflection or absorption profile can be obtained, and a reflection maximum or an absorption maximum is measured at the channel strip, which is, e.g., unprinted and white. The printed areas can also be recognized in the case of continuous scanning of the printed paper web, so that the printed information can also be detected by a sensor, and this information can also be used to detect the position of the paper web.
The detection of the printing ink-free or moistening agent-free channel strip can be performed, e.g., with a single detector with an appropriate light source and corresponding filters. It is also possible to perform absorption spectroscopy, in which case the spectrum is recorded in a certain range. The presence of the material to be detected can then be demonstrated from the absorption peaks and the absence of, e.g., moistening agent or of the ink can be determined.
Thermal radiation sources may be used as radiation sources. It is advantageous to use band pass filters in the spectral range of 2.8 xcexcm to 3 xcexcm, which may be arranged in front of the radiation source or the detector. Other possible radiation sources are infrared light-emitting diodes, which have an emission at about 2.9 xcexcm. It is also possible, in principle, to use laser light sources, e.g., erbium laser or lead salt laser diodes. For example, photoconductors, which are based on lead sulfide or lead selenide, may be used as detectors. It is possible, in principle, to use any detector that is sufficiently sensitive in the wavelength range and responds sufficiently rapidly. Thus, it is possible, in principle, to detect infrared radiation with thermally operating sensors, but these have a relatively long response time. If, e.g., a channel strip width of 10 mm and a web velocity of 10 m/sec are assumed, a remission maximum appears for a duration of 1 msec. This time becomes shorter when the measuring field of the sensor becomes too large. The sensor should therefore have a response time of less than 0.5 msec, and a response time of less than 200 xcexcsec is advantageous. The above-mentioned lead sulfide detectors have reaction times of about 200 xcexcsec. Lead selenide detectors have reaction times of about 200 xcexcsec. Lead selenide detectors have shorter reaction times, but are less sensitive.
If such a sensor is positioned, e.g., at a short distance in front of the entrance of the folding apparatus, the above-described recognition of the channel strip can be used to regulate the cutting position.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.